Various systems are currently used to spray a liquid onto a web. One example system includes a plurality of nozzles that are connected to a single manifold. The manifold is typically connected to a large tank of liquid (e.g., an adhesive) that is spaced some distance from the manifold. The large tank of liquid is pressurized such that liquid is delivered from the tank to the manifold, and then to the individual nozzles.
Another example system includes a plurality of pumps that are connected between a plurality of nozzles and a large tank of liquid. The flow of liquid from the large tank to the nozzles is typically controlled by one valve that is between the tank and one of the respective pumps, and another valve that is between the respective pumps and corresponding nozzles. The nozzles may be operated all together, or only groups of the nozzles may be operated, depending on the pattern that is placed on the web.
Existing systems are unable instantaneously to spray liquid at a specific point in time because there is a time lag between when a control signal is sent to open any nozzles, valves and manifolds and when the liquid is actually dispensed from the nozzle. The systems are similarly unable to instantaneously stop spraying liquid because there is a time lag between when a control signal is sent to close any nozzles, valves and manifolds and when the liquid is no longer dispensed from the nozzle. Delays in spraying the liquid, and stopping the spray of liquid, result in the pattern being sprayed out of position on a continuously moving web.
A compensation factor is typically estimated manually and built into such systems to account for the delays in starting and stopping the spray of liquid. The compensation factor is fixed such that one drawback with existing methods is that they fail to account for changes in length of the delay that it takes to turn the nozzles on and off. The length of the “on” delay and the “off” delay can vary over time due to such factors as wear of the nozzles and other components (among other factors). Another drawback with existing methods is that the compensation factor is typically the same for the “on” delay and the “off” delay even though during operation the “on” delay is often times different from the “off” delay.
There are some methods that include monitoring the relative location of one edge of a sprayed pattern on a web. The monitoring is done to provide information to a control system that modifies when the respective nozzles are turned on.
One drawback with some existing methods is that they are limited to controlling either the leading or trailing edge of an adhesive pattern on a moving web. As a result, existing systems are limited in their ability to accurately register (i.e., position) patterns of spray on a moving web.